A polyunsaturated fatty acid and derivatives thereof have many physiological activities such as reducing the fat level in blood, and have been used as a raw material for drugs, cosmetic preparations, food, and the like for many years. Methods of purifying a polyunsaturated fatty acid and derivatives thereof having high purity and excellent quality have been studied.
For example, a silver complex-forming technique has been known (Patent Documents 1 to 4). The silver complex-forming technique utilizes a property in which a polyunsaturated fatty acid and derivatives thereof become water-soluble as a result of forming a complex with a silver ion. Patent Documents 1 to 4 disclose that a silver salt that has been used to purify a polyunsaturated fatty acid and derivatives thereof can be recycled. However, a silver salt very easily deteriorates. When a polyunsaturated fatty acid and derivatives thereof are purified using a silver salt that has deteriorated, an excellent purified product cannot be obtained because impurities may be mixed, or a deterioration in flavor may occur. Therefore, the reality was that it was very difficult to recycle a silver salt. When industrially purifying a polyunsaturated fatty acid and derivatives thereof, it was necessary to prepare a new silver salt aqueous solution for each purification process. This significantly increases the purification cost. Therefore, technology that makes it possible to repeatedly recycle a silver salt aqueous solution over a long period has been desired to inexpensively provide a polyunsaturated fatty acid and derivatives thereof having excellent quality.